The present invention relates to pure-phase, crystalline tungstates of general composition Me(II)WO.sub.4, where Me(II)=Co, Ni, Zn, Pb, Mg, Ca, Sr and/or Ba, to a method of preparing them and to their use.
Among their other applications, metal tungstates are employed as starting materials for the preparation of complex perovskites (e.g. Pb.sub.2 MeWO.sub.6), which are used as materials for electrical ceramics and which are also used as a base material for X-ray intensifier phosphors. Methods are known for the synthesis of metal tungstates, particularly the mixing of metal oxides and subsequent solid state reaction at at least 750.degree. C. In this "mixed oxide" method, which is described in U.S. Pat. No. 5,079,199 for example, the oxides have to be homogenized for a long period of time.
This processing step is generally carried out by wet milling in ball mills over a long period of time. In the course of this operation, the oxide mixture naturally becomes contaminated with extraneous elements. When ceramic grinding media are used these contaminants mainly consist of the elements Zr, Al and Si. However, since the optimum homogenization is not obtained in this process step, the dried mixture has to be calcined for several hours at temperatures above 750.degree. C.
The principal object of the present invention is to provide metal tungstates which are not subject to the prior art disadvantages described above.
A further object of the invention is to prepare enhanced perovskites. A further object of the invention is to prepare enhanced X-ray intensifier phosphors.